I recently purchased the Exspect EX864 iPhone Headphone Adapter which basically extends the existing 4 pin headphone socket (speaker/microphone) for your iPhone. It is useful if you have e.g. a large iPhone case such as the Otterbox Defender and are looking to connect a 3rd party device that would otherwise be blocked by the case, such as the ithlete Receiver.
After testing the device I found it worked in terms of passing through earphone audio but not in terms of passing through the microphone. Further testing with a multimeter showed that the 4th ring (sleeve) which is the microphone connection was not passed through.
Since the microphone is the sleeve connection (the only one easily accessible on the socket) I was able to run a wire down the side of the case from the top socket to the 4th ring on the plug, solder it in place, and finish it off with some heat proof tubing. It seems to work fine in terms of fixing the microphone problem.
Note: I also purchased the iFrogz Fritz Headphone Adapter which had the same problem as the EX864 in terms of the microphone connection not being wired through.